It is common in certain applications to have curved or formed glass sheets with an opaque border portion. The border portion is commonly a paint that is applied to the glass sheet prior to the glass sheet being heated so that it can be formed by a forming die. A problem in the art has been the sticking of the painted surface to the forming die during the forming process.
Commonly the paints used are ceramic paint compositions generally well known to the skilled artisan. These ceramic paint compositions can be used, for example, to form borders around the edges of glass sheets which are used as windshields, sidelites and backlites in motor vehicles.
Such ceramic paints usually include a mixture of metal oxides which together act as a coloring agent for the ceramic paint. The metal oxides are nonreactive with one another and nonreactive with any elements or compounds they come in contact with while being heated to temperatures up to about 1300.degree. F. The mixture of metal oxides can be controlled so as to get a selected color from the ceramic paint composition. Normally, in automotive applications, the selected color is black although shades of gray are now becoming popular.
Such ceramic paint compositions also include a glass frit which melts generally at a temperature well below 1350.degree. F. The glass frit is the material which bonds the mixture of metal oxides to the glass sheet and ensures that the mixture of metal oxides remains after the glass sheet has been cooled back to room temperature.
A vehicle is normally mixed with the metal oxides and glass frit to allow the ceramic paint composition to be applied in a paint application process. For example, if the paint application process is a silk screen printing operation, the vehicle can be an oil-base or a UV-base organic medium for carrying the other materials during the silk screen printing operation.
In my U.S. Pat. No. 4,596,590, which issued on June 24, 1986, I disclosed a method of forming a glass sheet with a ceramic paint thereon. My patent contains a discussion of a problem encountered in the sticking of ceramic paint to a forming die covered by fiberglass when the forming die engages a heated glass sheet with ceramic paint thereon.
My patent discloses a method of forming a glass sheet with a ceramic paint thereon which has the following steps. A ceramic paint is applied to the glass sheet. The ceramic paint includes normal ceramic paint components along with, as a new component thereof, a metal oxide having at least a low valance oxidation stats and a high valance oxidation state. The metal oxide is in its low valance state when applied to the glass sheet as part of the ceramic paint. The glass sheet is heated to a temperature which softens the glass sheet sufficiently so that the glass sheet can be formed. The glass sheet and the ceramic paint thereon are engaged with a fiberglass covered forming die to form the heated glass sheet to a desired shape. The forming die is removed from engagement with the glass sheet. The metal oxide addition to the ceramic paint in its low valance state is credited with being effective to prevent sticking of any of the ceramic paint to the forming die. The glass sheet is cooled to obtain a formed glass sheet with a ceramic paint thereon.
In both U.S. patent application Ser. No. 936,575, filed Dec. 1, 1986, now U.S. Pat. No. 4,684,388, entitled "Method of Forming a Glass Sheet with a UV-Base Ceramic Paint Thereon, and U.S. application Ser. No. 936,576, filed Dec. 1, 1986, now U.S. Pat. No. 4,684,389, entitled "Method of Forming a Glass Sheet with an Oil-Base Ceramic Paint Thereon", I disclosed that the method proposed in my U.S. Pat. No. 4,916,590 when using stannous oxide as the metal oxide having at least a low valance oxidation state and a high valance oxidation state, did have some drawbacks. I found that in a UV-base system, when stannous oxide is present in the paint in an amount greater than 20% by weight or so, there is some tendency for the paint to stick to fiberglass when a relatively high pressure is applied to the painted glass sheet in a bending operation.
When working with an oil-base ceramic paint, I also found that the use of stannous oxide as the metal oxide having at least a low valance oxidation state and a high valance oxidation state also had some drawbacks. I found that stannous oxide did, in some cases, oxidize too rapidly when subjected to a heating operation. This rapid oxidation caused a fine, white dust to be generated. This dust has a tendency to move over the surface of the glass sheet during the bending operation. The loose dust, at an elevated temperature, can cause marking and/or indentation of the glass sheet, which results in a scrappage thereof.
In my application Ser. No. 936,575, now U.S. Pat. No. 4,684,388, I disclosed a method of forming a glass sheet with a UV-base ceramic paint thereon, the method having the following steps. A UV-base ceramic paint is applied to the glass sheet. The UV-base ceramic paint includes normal ceramic paint components along with, as a new component thereof, finely divided zinc metal powder. The glass sheet is subjected to UV radiation and then heated to a temperature which softens the glass sheet sufficiently so that the glass sheet can be formed. The glass sheet and the ceramic paint thereon are engaged with a fiberglass covered forming die to form the heated glass sheet to a desired shape. The forming die is removed from engagement with the glass sheet. The finely divided zinc metal powder addition to the ceramic paint is credited with being effective to prevent sticking of any of the ceramic paint to the forming die. The glass sheet is cooled to obtain a formed glass sheet with a ceramic paint thereon.
In my application Ser. No. 936,576, now U.S. Pat. No. 4,684,389, I disclosed a method of forming a glass sheet with an oil-base ceramic paint thereon, the method having the following steps. An oil-base ceramic paint is applied to the glass sheet. The oil-base ceramic paint includes normal ceramic paint components along with, as a new component thereof, finely divided zinc metal powder. The glass sheet is heated to a temperature which softens the glass sheet sufficiently so that the glass sheet can be formed. The glass sheet and the ceramic paint thereon are engaged with a fiberglass covered forming die to form the heated glass sheet to a desired shape. The forming die is removed from engagement with the glass sheet. The finely divided zince metal powder addition to the ceramic paint is credited with being effective to prevent sticking of any of the ceramic paint to the forming die. The glass sheet is cooled to obtain a formed glass sheet with a ceramic paint thereon.
Despite the methods that I have developed which have been described above, there are situations where paint sticking still occurs when high pressures are applied on a ceramic paint by a fiberglass covered die in which there is a high degree of bend over a relatively short distance. Under such conditions, I have found that even though zinc powder is added to the ceramic paint, a degree of sticking of the ceramic paint to the fiberglass cloth will take place in the areas of bending where high pressures over small areas are used between the fiberglass covered forming die and the painted glass sheet.
It is a principal object of the method of my invention to provide a method of forming a glass sheet with ceramic paint thereon which can be carried out at high contact pressures between a fiberglass covered forming die and ceramic paint without the sticking of ceramic paint to the fiberglass. The method which I disclose herein has been successfully developed for forming glass sheets with ceramic paint thereon under high pressure over small areas of such glass sheets without any sticking of ceramic paint to the fiberglass covered forming dies.
The most relevant prior art that I am aware of is the material discussed above. No search was conducted on the subject of this invention in the U.S. Patent and Trademark Office or in any other search facility.